1. Field of the invention
This disclosure relates to a compact, high collection efficiency scintillator used to obtain high-resolution images of devices in FIB (Focused Ion Beam) systems.
2. Description of the Related Art
Over the past two decades, liquid metal ion source (LMIS) based focused ion beam (FIB) systems have found numerous applications in various branches of science and industry. For example, within the semiconductor industry, focused ion beam systems are used for integrated circuit (IC) device modification, failure analysis, probe point creation, photomask repair, maskless lithography, TEM sample preparation, scanning ion microscopy and secondary ion mass spectroscopy. In many of these applications, there is a need to obtain high-resolution FIB images of the device on which the FIB is operating. The images allow a determination of the affect of beam on the device. For example, obtaining high quality cross-sectional images of a semiconductor IC device is essential for identifying process defects. In general, there are two important parameters that jointly affect the FIB image quality:
(1) the spot size of the ion beam which, in mm, is determined by the ion column optics, and
(2) the collection efficiency of the secondary electron detection system, which affects the image contrast level. (The secondary electrons are those that come off the device as a result of the incident FIB.)
It is well known that the image resolution improves with decreasing spot size of the ion-beam. It is also known that the spot size of the ion beam varies inversely as the beam current. As a result, higher image resolutions can be obtained by using smaller beam currents. Typically, ion beam currents of 1 pA or lower give rise to high resolution images that compensate for the low signal-to-noise ratios (SNR) that is inherently associated with small beam currents. Also, according to Orloff et al. if the Rayleigh criterion is used to quantify the image resolution, then there is a direct correlation between the image resolution and the image contrast level. The image contrast level is, in turn, directly dependent on the number of secondary electrons actually collected by the secondary electron detection system. Scintillators collecting electrons at the lowest frequencies with highest probability are most effective, since the most probable secondary electron energies are around 0-5eV . See Goldstein, et al., xe2x80x9cScanning Electron Microscopy and X-Ray Microscopyxe2x80x9d, 2nd Edition, Plenum, 1992.
Even though it is difficult to quantify the dependence of the image resolution on the secondary electron collection efficiency, possibly the secondary electron collection efficiency is indeed a limiting factor with regard to image resolution. It is therefore of great practical interest to incorporate secondary electron detectors having high collection efficiency.
A common type of the secondary electron detector used in FIB systems is the Everhart-Thornley type. The Everhart-Thomley type scintillators have been used for many years in charged particle (ion or electron beam) instruments for collecting secondary electrons. This design typically consists of a flat scintillator that emits photons in proportion to the number of electrons striking it, a light pipe to deliver the photons to the photoelectron multiplier tube, and a cap or mesh on which a positive bias voltage is normally applied to increase the collection efficiency. U.S. Pat. No. 4,588,890 titled xe2x80x9cApparatus and method for composite image formation by scanning electron beamxe2x80x9d discusses the Everhart-Thoruley detector and is incorporated herein by reference.
However, the present inventors have identified several deficiencies in these prior art scintillators.
The present inventors have recognized that in the past, the spatial orientation of the collector relative to the sample and primary beam column was not optimized through range. The compactness of the whole assembly was also not emphasized. In FIB systems, many practical considerations, such as vacuum pumping speed and mechanical interference with gas injectors and mechanical probes, require that the scintillator assembly be as small as possible without sacrificing the secondary electron collection efficiency. Also, we determined that applying a bias voltage could have an influence on the primary ion beam as well as other sensors inside the chamber.
We optimized in one embodiment the position, the angle of the scintillator disk relative to the sample and the column, and the shape of the ground cap, to achieve 100% collection efficiency using a very compact scintillator assembly, with typical bias voltage.
The present scintillator has optimal geometry, orientation, and overall size to achieve near 100 percent collection efficiency for the majority of useful secondary electron energy ranges. Further, the scintillator may be placed closer to the primary ion beam, which further improves collection efficiency. This greatly improves the quality of images produced and the reliability of the system.
The present scintillator avoids primary ion beam deflection fluctuations, which affect the normal FIB imaging, cutting and deposition operations. These fluctuations result from accumulated charge on the insulator between the scintillator disk and ground cap reaching a certain threshold and subsequent discharge or arcing. As a result, the spatial electrical potential distribution also changes suddenly. To solve the problem of insulator charge-up, the present scintillator provides ultra-high collection efficiency and is also much less susceptible to charging. The insulator is placed such that it is completely out of the secondary electron path. At the same time, the gap between the grounding cap and the high voltage scintillator disk and its holding ring is increased. This allows collection efficiency, and at the same time, improves the system reliability significantly.